Dynamic valve control in a skip fire controlled engine

ABSTRACT

Various methods and arrangements for improving fuel economy and noise, vibration, and harshness (NVH) in a skip fire controlled engine are described. An engine controller dynamically selects a gas spring type for a skipped firing opportunity. Determination of the skip/fire pattern and gas spring type may be made on a firing opportunity by firing opportunity basis.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the priority of U.S. Provisional PatentApplication No. 62/508,020 (P064P), filed May 18, 2017. This applicationis also a Continuation-in-Part of U.S. patent application Ser. No.15/171,931 (P052), filed Jun. 2, 2016 and Ser. No. 15/282,308 (P056),filed Sep. 30, 2016. U.S. patent application Ser. No. 15/282,308 in turnclaims priority of Provisional Application No. 62/353,772, filed Jun.23, 2016. Each of the foregoing priority applications is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to operation of an internalcombustion engine under skip fire control. Various embodiments relate tochanging the timing of cylinder intake and exhaust events to formdifferent types of gas springs within the engine's cylinders.

BACKGROUND

Most vehicles in operation today (and many other devices) are powered byinternal combustion (IC) engines. An internal combustion enginetypically has a reciprocating piston which oscillates within a cylinder.Combustion occurs within the cylinder and the resulting torque istransferred by the piston through a connecting rod to a crankshaft. Fora four-stroke engine, air, and in some cases fuel, is inducted to thecylinder through an intake valve and exhaust combustion gases areexpelled through an exhaust valve. In typical engine operation, thecylinder conditions vary in a cyclic manner, encountering, in order, anintake, compression, expansion, and exhaust stroke in a repeatingpattern. Each repeating pattern may be referred to as a working cycle ofthe cylinder.

Internal combustion engines typically have a plurality of cylinders orother working chambers in which an air-fuel mixture is combusted. Theworking cycles associated with the various engine cylinders aretemporally interleaved, so that the expansion stroke associated with thevarious cylinders is approximately equally spaced, delivering thesmoothest engine operation. Combustion occurring in the expansion strokegenerates the desired torque as well as various exhaust gases. Theexpansion stroke is often denoted as the combustion or power stroke,since this is the power producing stroke.

Under normal driving conditions, the torque generated by an internalcombustion engine needs to vary over a wide range in order to meet theoperational demands of the driver. Over the years, a number of methodsof controlling internal combustion engine torque have been proposed andutilized. Some such approaches contemplate varying the effectivedisplacement of the engine. Two different engine control approaches thatvary the effective displacement of an engine include: (1) the use ofmultiple fixed displacements; and (2) skip fire engine operation. Infixed multiple displacement control some fixed set of cylinders isdeactivated under low load conditions; for example, an 8-cylinder enginethat can operate on the same 4 cylinders under certain conditions. Incontrast, skip fire control operates by sometimes skipping and sometimesfiring a cylinder. In some engines all cylinders are capable of firingor skipping, while in other engines only a subset of the engine'scylinders have skip fire capability. In general, skip fire enginecontrol is understood to offer a number of potential advantages,including the potential of significantly improved fuel economy in manyapplications. Although the concept of skip fire engine control has beenaround for many years, and its benefits are understood, skip fire enginecontrol has only recently obtained some commercial success.

It is well understood that operating engines tend to be the source ofsignificant noise and vibrations, which are often collectively referredto in the field as NVH (noise, vibration and harshness). In general, astereotype associated with skip fire engine control is that skip fireoperation of an engine will make the engine run significantly rougher,that is with increased NVH, relative to a conventionally operatedengine. In many applications, such as automotive applications, one ofthe most significant challenges presented by skip fire engine control isvibration control. Indeed, the inability to satisfactorily address NVHconcerns is believed to be one of the primary obstacles that hasprevented widespread adoption of skip fire types of engine control.

U.S. Pat. Nos. 7,954,474, 7,886,715, 7,849,835, 7,577,511, 8,099,224,8,131,445, 8,131,447, 8,616,181, 8,701,628, 9,086,020 9,328,672,9,387,849, 9,399,964, 9,512,794, 9,745,905, and others, describe avariety of engine controllers that make it practical to operate a widevariety of internal combustion engines in a skip fire operational mode.Each of these patents and patent applications is incorporated herein byreference. Although the described controllers work well, there arecontinuing efforts to further improve the performance of these and otherskip fire engine controllers to further mitigate NVH issues and improvefuel economy in engines operating under skip fire control. The presentapplication describes additional skip fire control features andenhancements that can improve engine performance in a variety ofapplications.

SUMMARY

In various embodiments, a system and method to vary the type of gasspring in a skipped working chamber of a skip fire controlled engine isdescribed. Various embodiments relate to changing the timing of cylinderintake and exhaust events to modify the nature of gases trapped in theworking chamber during a skipped working cycle. The fuel injectiontiming and ignition timing may also be modified. The nature of the gasspring can be varied to improve NVH levels and fuel economy. Oilconsumption and exhaust emissions may also be considered indetermination of the gas spring type.

In one aspect, a method of controlling the nature of the gases trappedin a working chamber during a skipped working cycle is described.Depending on the intake and exhaust valve opening and closing sequence,the gases trapped have different masses and constituent components,effectively forming gas springs during the skipped working cycle(s). Askipped working cycle may have a low pressure exhaust spring (LPES), ahigh pressure exhaust spring (HPES), or an air spring (AS). The type ofgas spring may be chosen to optimize fuel efficiency and provide anacceptable level of NVH.

In another aspect, an engine controller determines a fire/skip sequenceappropriate for delivering a requested engine output. The fire/skipsequence includes information on the type of gas spring present inskipped working cycles and the timing available to deactivate valves.

In another aspect, a method of operating an engine in response to a notorque request is described. All working chambers of the engine areskipped in response to the no torque request. During the duration of thedeactivation, each working chamber operates with an air spring or highpressure exhaust spring type gas spring for a least one working cycle ofthe skipped working cycles.

The various aspects and features described above may be implementedseparately or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a schematic example diagram showing a portion of an enginesystem.

FIG. 2 is a representative graph of the pressure vs. volume relationshipin a cylinder over a working cycle.

FIG. 3 is a representative graph of the pressure vs. volume relationshipin a cylinder for a LPES type gas spring.

FIG. 4 is a representative graph of the pressure vs. volume relationshipin a cylinder for a HPES type gas spring.

FIG. 5 is a representative graph of the pressure vs. volume relationshipin a cylinder for an AS type gas spring.

FIG. 6 is a representative graph showing torque signatures from AS,HPES, LPES type gas springs associated with a single cylinder.

FIG. 7 is a representative graph of the total engine torque signaturefor a skip fire controlled, eight-cylinder engine operating at a firingfraction of ½ with a LPES and HPES type gas spring.

FIG. 8 is a schematic example diagram showing an exemplary enginecontrol system.

FIG. 9 is a plot of the brake torque versus intake manifold absolutepressure for a LPES and AS type gas spring for a representative engine.

FIG. 10 is a plot of the brake torque versus mass air charge for a LPESand AS type gas spring for a representative engine.

FIG. 11 is a plot of the brake specific fuel consumption versus brakemean effective pressure for a LPES and AS type gas spring for arepresentative engine.

In the drawings, like reference numerals are sometimes used to designatelike structural elements. It should also be appreciated that thedepictions in the figures are diagrammatic and not to scale.

DETAILED DESCRIPTION

The present invention relates to improving operation of a skip firecontrolled internal combustion engine that provides motive torque forpowering a vehicle. The present invention discloses that by changing thetiming of induction and exhaust events the gases trapped in a workingchamber or cylinder during a skipped firing opportunity may differ. Thetrapped gases form different types of “gas springs” and the type of gasspring used may vary depending on the cylinder load, engine speed, andskip fire pattern. “Gas spring type” here refers to the amount and typeof gas trapped in a deactivated cylinder. Several types of cylinderdeactivation strategies can be used in skip fire operation. Thedeactivation strategies can generally be categorized into three types:low pressure exhaust spring (LPES), high pressure exhaust spring (HPES),and air spring (AS). Each category varies the order and timing ofdeactivation/reactivation of intake valves, exhaust valves, fuelinjection, and spark timing. Within each of these gas spring types thereare various sub-categories described below that vary based on whichvalve starts or ends the skipping sequence.

FIG. 1 illustrates an example internal combustion engine that includes acylinder 161, a piston 163, an intake manifold 165, and an exhaustmanifold 169. Air is inducted into cylinder 161 through an intake valve185. Combustion gases are vented from cylinder 161 through an exhaustvalve 187. A throttle valve 171 controls the inflow of air from an airfilter or other air source into the intake manifold 165. Expanding gasesfrom combustion increase the pressure in the cylinder and drive thepiston down. Reciprocal linear motion of the piston is converted intorotational motion by a connecting rod 189, which is connected to acrankshaft 183. A 4-stroke engine takes two crankshaft revolutions, 720degrees, to complete a working cycle.

The present invention relates generally to methods and devices forcontrolling the operation of intake and exhaust valves of an internalcombustion engine during skip fire operation. In various embodiments,the valves are controlled using an eccentric cam to open and close thevalves. A collapsible valve lifter may be incorporated in the valvetrain to allow deactivation of the valves during a skipped workingcycle. That is the valve will remain closed as long as the collapsiblelifter associated with the valve is in its collapsed state and will openand close with cam rotation when the lifter is in its rigid state.Collapsible lifters are one form of a general class of lost motionsystems where cam rotation does not result in valve motion. Valvedeactivation may alternatively be controlled by a sliding cam,collapsible lash adjuster, or collapsible roller finger follower, amongother methods. Valve timing may be controlled with a cam phaser thatadjusts the opening/closing time of the valve relative to the crankangle. In some embodiments, no cam is required to move the valves. Thevalve motion may be accomplished by electro-magnetic, hydraulic, orpneumatic means. Any of these valve motion systems may be used with thepresent invention.

In one embodiment the combustion and gas exchange processes of aninternal combustion engine operating in skip fire mode may be changed sothat the net or total torque generation characteristics are modified. Inparticular, the gas spring type of deactivated cylinders may be modifiedto adjust the temporal torque profile. Proper timing of the gas springinduced torque with respect to the cylinder firing events may result inan overall engine torque waveform with desirable characteristics. Forexample, there may be less content at frequencies most likely to beperceived as vibration or noise by vehicle occupants. The cylinder mayalso operate at a higher efficiency depending on the type of gas springand cylinder load.

FIG. 2 depicts the pressure vs. volume (PV) relationship for a naturallyaspirated, 4-stroke, Otto cycle engine over a firing working cycle. Itis noted that the vertical (pressure) and horizontal (specific volume)axes are represented on a log scale in this figure and in the PVdiagrams of FIGS. 3-5. Normal engine operation involves a repeatingcycle of intake, compression, expansion, and exhaust that occurs overfour strokes of piston movement, or two rotations of the crankshaft.Each stroke of piston movement from top dead center (TDC) to bottom deadcenter (BDC), or vice versa, corresponds to one stroke or 180 degrees ofcrankshaft rotation. An air intake or induction stroke occurs during thefirst stroke of piston movement from TDC to BDC, from point A to point Bin FIG. 2. A compression stroke occurs during the second stroke ofpiston movement from BDC to TDC, from point B to point C in FIG. 2.Combustion may be initiated by a spark ignition and occurs around TDCnear the end of the second stroke and beginning of the third stroke. Anexpansion stroke occurs during the third stroke from TDC to BDC, frompoint C to point D in FIG. 2. An exhaust stroke occurs during the fourthstroke from BDC to TDC, from point D returning to point A in FIG. 2. ThePV curve forms two loops. In general, the area bounded by the upper looprepresents the amount of work that is generated by combustion in afiring cylinder, whereas the area bounded by the lower loop representsthe energy losses that are experienced due to pumping air into and outof the cylinder (these losses are frequently referred to as pumpinglosses). Also shown in FIG. 2 is the atmospheric pressure, denoted asP_(atm). For most of the intake stroke the cylinder pressure is belowatmospheric pressure, since the cylinder is inducting air from an intakemanifold whose pressure is held below atmospheric pressure by control ofa throttle valve.

For improved fuel efficiency, it is desirable to make the pumping lossesas small as possible. This is achieved by opening the throttle, whichshifts the portion of the PV curve between points A and B closer toatmospheric pressure. Reducing pumping loss is a primary reason skipfire operation offers improved fuel efficiency, since the engine outputis controlled primarily by firing density, not by throttling air flowinto the engine.

FIG. 2 also depicts representative opening and closing times for theintake and exhaust valve(s) of the cylinder. Point 1 corresponds to theopening time of the intake valve. Point 2 corresponds to the closingtime of the intake valve. Point 3 corresponds to the opening time of theexhaust valve. Point 4 corresponds to the closing time of the exhaustvalve. As shown the intake valve opens a bit before TDC and the exhaustvalve closes a bit after TDC resulting is some intake/exhaust valveoverlap, which is typical in a modern engine. Point 2, where the intakevalve closes, typically occurs a little after BDC to take advantage ofgas momentum into the cylinder to squeeze more air into the cylinder andincrease volumetric efficiency. Point 3, where the exhaust valve opens,typically occurs a bit before BDC. It should be appreciated thatvariable cam timing and valve lift strategies may shift intake/exhaustvalve opening/closing either before or after TDC or BDC. Fuel injectioncan be either directly into the cylinder (direct injection, DI) orindirectly into the cylinder by injecting fuel into the incoming aircharge outside the cylinder (port fuel injection, PFI) and will occurbefore the intake stroke (Point A) for PFI methods and during the intakeand compression strokes (Point A to past Point B) for DI methods. Sparkignition, which triggers combustion, occurs around the end of thecompression stroke as noted in FIG. 2. Other control strategies arepossible, such as recompression where air inducted at the beginning ofone working cycle is retained in the cylinder thru that cycle andcombusted or vented in some subsequent cycle.

When a cylinder is skipped or deactivated, rather than fired, over aworking cycle the PV curve is different than that depicted in FIG. 2. Inparticular for engines capable of intake and/or exhaust valvedeactivation different amounts and types of gas can be trapped in acylinder during a skipped working cycle forming different types of gassprings.

One type of gas spring is a low pressure exhaust spring (LPES), whose PVcharacteristics are depicted in FIG. 3. An LPES gas spring is realizedby deactivating the intake valve immediately on an induction stroke thatfollows an exhaust stroke after a combustion stroke. In this case theintake valve remains closed whereas it would normally open for anotherinduction event resulting in the cylinder never being open to intakemanifold vacuum. Rather than the PV curve dropping below atmosphericpressure, the PV curve stays at or slightly above atmospheric pressureat point A′. Since the enclosed cylinder volume is small at the end ofthe exhaust stroke when the cylinder is sealed (Point 4′), use of a LPESleads to very low in-cylinder pressures at the end of the intake strokewhen the enclosed cylinder volume is maximized (Point E). The cylindercontains mostly residual exhaust gas from the previous cycle. Thetrapped residual exhaust gases then experience a compression strokemoving back along the PV curve to at or near point A′. No fuel isinjected during the compression stroke. Spark may or may not occur nearTDC; however, there would be no combustion because there is nocombustible air or fuel in the cylinder. No energy release would occurdue to the absence of fuel and fresh charge, and the piston would startan expansion stroke moving from at or near point A′ back to point E. AtBDC, the exhaust valve would be deactivated and the piston wouldrecompress the mixture during the exhaust stroke moving from point Eback to at or near point A′. All valves would remain deactivated andfuel injection would not occur for as long as desired.

Practically, the LPES peak pressure would slowly increase untilthermodynamic equilibrium was achieved. Depending upon crankcasepressure and combustion chamber pressure, vapors from the crankcase mayflow from the crankcase around the piston rings and into the cylinder,increasing the mass and pressure of gas enclosed in the cylinder volume.When the decision to reactivate that cylinder is made, one option is toreactivate the intake valve first, causing the mixture of exhaustresidual gas and crankcase vapor in the cylinder to be augmented withfresh air charge. Fuel injection and spark, if necessary, arereactivated, and combustion resumed. Finally, the exhaust valve isreactivated and the cylinder is back in normal firing mode.

A variant on the LPES control cylinder venting method is LPES withre-exhaust. In this case, the exhaust valve is reactivated before theintake valve. This results in two exhaust strokes without an interveninginduction stroke. In this reactivation strategy the exhaust valve isreactivated first, followed by the intake valve and then fuel and spark.There are several reasons for doing this. First by having a re-exhaustevent, gases that have leaked into the cylinder may be expelled prior toinduction, making the inducted charge more similar to that of a cylinderoperating without deactivation. A normally firing engine relies on valveoverlap and gas flow momentum to scavenge as much exhaust residual fromthe cylinder as possible. This is missing from LPES without re-exhaustand will lead to lower volumetric efficiency on the first reactivatedcycle. Second, in the event that combustion has occurred, perhapsmistakenly, during a skipped cycle, the re-exhaust would prevent theintake valve from opening on a HPES and causing potential valve traindamage. Re-exhaust could be incorporated into a safety feature thatrequires the exhaust valve of any cylinder to open before the intakevalve is allowed to open. If the exhaust valve fails to open or aredeactivated, the intake valve would automatically be deactivated. Adownside of this method is that its pumping loop is larger, and thusenergy efficiency is lower, than that of normal LPES if the number ofskipped cycles is short. As the number of skipped cycles increases theperformance of the two methods becomes essentially equivalent, sincemost strokes experience identical conditions.

A second type of gas spring is a HPES, high pressure exhaust spring,whose PV characteristics are depicted in FIG. 4. When deactivating acylinder via the HPES method, the induction, compression, and expansionstrokes occur normally, and the exhaust stroke is skipped bydeactivating the exhaust valve prior to point D, preventing the exhaustvalve opening 3 and trapping high pressure exhaust gas in the cylinder.The high pressure exhaust gas would then be recompressed by the cylindermoving from point D to point F during the exhaust stroke, compressingthe in-cylinder gases back to peak pressures somewhat higher than thoseencountered in the previous expansion stroke. The intake valve would bedeactivated during the subsequent intake stroke, preventing possiblevalve train damage that could occur from opening a valve on such highpressure. The piston would then expand the high pressure gas a secondtime while it moves from point F to point G during what would be theintake stroke. Fuel injection would be disabled at this point so thecompression and expansion strokes would just compress and expand thetrapped high pressure exhaust gases. The compression and expansion ofthe high pressure gas would continue for as long as the cylinder isskipped, ideally between the same two pressures at TDC and BDC. Inreality, heat and mass transfer from the cylinder would cause thecylinder pressure to drop rapidly, so each successive pressure/volumetrajectory would be somewhat lower. When the decision is made toreactivate the cylinder, the exhaust valve may be reactivated first,followed by the intake valve, and finally fuel injection. This wouldallow the high pressure gas to be exhausted normally, permit a freshcharge to enter the cylinder, and normal combustion on the next firing.

A variation of the HPES control method is HPES with re-fueling. A maindifference is that once the decision is made to reactivate a skippingcylinder, first the fuel is reactivated, then exhaust and intake. Thisrequires DI and assumes enough combustible charge has either leaked intothe cylinder from the crankcase or remains in the residual that has yetto be exhausted since the last combustion event. This variant isparticularly applicable to a lean burn engine, where significant levelsof excess oxygen may remain in the trapped residuals.

Another variation of the HPES control method is HPES with re-intake. Inthis strategy, when a skipping cylinder is reactivated, the intake valveis reactivated first, followed by fuel and exhaust. This is similar tothe reactivation process of LPES without re-exhaust. As mentioned above,this strategy has the potential to cause serious valve train damage dueto opening the intake valve on a cylinder pressure at a level nearcombustion peak pressure. This can be prevented with appropriate designof the intake valve and its associated valve train. By opening theintake valve on a HPES, the high pressure exhaust residual in thecylinder will blow down into the intake manifold, causing significantheating of the incoming charge. Volumetric efficiency may be low on thefirst reactivated cycle. The intake valves, ports, and manifold wouldneed to be designed to handle higher than usual levels of pressure andtemperature. This method has very large expansion/compression lossesduring deactivation resulting in large negative spring mean effectivepressure, and consequently low fuel efficiency if the number of skippedcycles is short. This mode of operation may be especially useful inengines where some working cycles use homogenous charge compressionignition (HCCI) or similar types of combustion strategies.

A third type of gas spring is an AS, air spring, whose PVcharacteristics are depicted in FIG. 5. In the AS method of cylinderdeactivation, the first stroke to be altered when deciding to deactivateis the expansion stroke. In a DI engine, intake would occur normally,but fuel would not be injected. The piston would compress the charge ofmostly air with no combustion, traveling from point B to point H. Frompoint H the air would expand to point I. At point I the exhaust valvewould be deactivated and the piston would compress the air a secondtime. At TDC the intake valve would be deactivated, and the piston wouldexpand the air a second time. This compression and expansion of mostlyair would continue for as long as the cylinder is desired to be skipped.Ideally, the cylinder would act like an air spring. Practically, heatand mass transfer from the cylinder to the surroundings cause thecylinder mass and pressure to decrease over time, so each successivepressure/volume trajectory would be somewhat lower until the averagepressure over a stroke is approximately atmospheric pressure. When thedecision to fire the cylinder is made, fuel injection and spark, arerestored first on a DI engine. This would likely produce a lower torquecombustion event due to a lower air charge stemming from gas leakage aswell as lower charge motion which normally aids fuel evaporation andmixing. The operation of the exhaust and intake valves would be restorednext, respectively, and the cylinder would resume normal operation.

A variant of an air spring is a fuel/air spring. In an PFI engine,intake and fuel injection would occur normally, but spark would bedeactivated when the decision to skip is made. The AS would be more likea fuel/air spring in this scenario, since the inducted gas is anair/fuel mixture in a PFI engine. In the absence of auto-ignition, themass and pressure of the fuel/air mixture in the cylinder would againdecrease over time as mass and heat loss to the crankcase and cylinderwalls, respectively, would still occur. Exhaust and intake valves aredeactivated in the same manner as for the DI engine. When the decisionto fire again is made, the spark would be restored. Again, a weakcombustion event would likely occur and the exhaust and then intakevalves would be reactivated.

In another variant of AS, AS with re-intake, both DI and PFI engineswould disable fuel injection first when the decision to skip is made.Intake would still occur normally, but no combustion would occur in theabsence of fuel and perhaps spark. Exhaust would be deactivated, andfinally, intake also would be deactivated and the engine would run in ASmode until the decision to reactivate is made. Once that happens, thefirst step is to reactivate the intake valve with fuel injection. Thiswould refill the cylinder with fresh charge. Spark would be enabled, ifnecessary, and normal combustion would occur. Finally, the exhaust valvewould be reactivated. This strategy has the benefit of avoiding thelower combustion air charge associated with normal AS. The downside isthat the pumping loop can be quite large if the number of skipped cyclesis short.

An AS with re-exhaust method differs from the other AS methods in thatwhen the decision is made to stop skipping and start firing again, theexhaust valve is reactivated first, followed by intake and fuelinjection. Like AS with re-intake, this avoids the lower air charge andresulting weak combustion event that would occur on the firstreactivated cycle. Unlike AS with re-intake, AS with re-exhaust canavoid the large pumping loop if the number of skipped cycles is short.However, this method pumps uncombusted air into the exhaust. If only onecycle is skipped, this method essentially never deactivates the valvesand a significant amount of air is pumped thru the cylinder, which mayimpact exhaust emission control systems.

The temporal torque profiles or signatures of the various types of gassprings are significantly different. FIG. 6 shows representative torquesignatures from AS, HPES, and LPES type gas springs associated with asingle cylinder. Also shown for comparison is the torque signatureassociated with a firing cylinder 610. The magnitude of the torqueresulting from the high-pressure gas spring 620 is very high, comparableto or higher than the torque magnitude from a firing cylinder, both in apositive and negative direction. The low-pressure spring torquesignature 630 has a very small magnitude, because of the very smallamount of gas trapped in the cylinder. For much of a working cycle thetrapped gas is at sub-atmospheric pressure as noted in the discussion ofFIG. 3. The magnitude of the torque associated with an air spring 640 isintermediate between the HPES and LPES cases, although it is generallycloser to the LPES case, since no hot combustion gases are trapped inthe cylinder.

The air charge associated with an induction stroke following a skippedfiring opportunity varies with the gas spring type that preceded theinduction event. The amount of residual charge during a following cycleis a function of whether intake and exhaust valves are opensimultaneously, and the relative timing of each, among other parameters.In addition, the relative temperature of the residual charge variesbased on a number of factors, including the amount of time the residualcharge is present in the combustion chamber and leakage into and out ofthe cylinder. Furthermore, pressure oscillations in the intake andexhaust manifold differ based on when and whether intake and exhaustvalves open and close in both a given cylinder and other enginecylinders. From these and other effects, combustion parameters such asinjected fuel mass and spark timing may be adjusted to compensate tomatch the inducted air charge. Injected fuel mass may be controlled toprovide a stoichiometric or near stoichiometric air/fuel ratio, which isnecessary for efficient operation of some pollution control devices,such as a 3-way catalyst. Spark timing may be adjusted to produce themaximum brake torque possible from the air/fuel charge, optimizing fueleconomy.

Combining torque signatures of each cylinder, properly phased accordingto the cylinder firing order, results in a summed torque signature.Depending on the number of cylinders in the engine each firingopportunity will be separated by a nominal amount of crank rotation. Forexample, for a 6-cylinder engine successive firing opportunities in anengine with equal spacing between firing events will occur nominallyevery 120°. For an 8-cylinder engine with equal spacing between firingevents, successive firing opportunities will occur nominally every 90°.For a 4-cylinder engine with equal spacing between firing events,successive firing opportunities will occur nominally every 180°. Thesecrank angles are nominal values only, as the timing of the spark firingmay be adjusted individually or in combination to change the profile ofthe resultant torque signature and optimize fuel economy.

FIG. 7 shows a representative resultant torque signature for an 8cylinder, 4-stroke engine operating with an alternatingskip-fire-skip-fire . . . pattern. One trace 720 shows a HPES torquesignature for the deactivated cylinders, and the other trace 730 shows aLPES torque signature for the deactivated cylinders. As can be seen thetorque signatures are significantly different, with the LPES mode havingsignificantly less deviation about a mean engine torque. Since NVH isgenerally correlated with the smoothness of engine torque delivery, FIG.7 suggests that for this firing pattern operating in an LPES mode ispreferable to HPES operation; other considerations, such as fueleconomy, being equal. Generally, the gas spring type, along with theoperational firing fraction or firing sequence, may be chosen to smoothvariations in the temporal torque profile below an acceptable level.Also, the gas spring type may be chosen to reduce spectral content inthe torque signature in a designated frequency range. This designatedfrequency range may correspond to a range of particular occupantsensitivity; for example, 0.5 to 12 Hz. It may also be chosen to avoidexciting any resonances in the vehicle or vehicle components.

Referring next to FIG. 8, a skip fire engine controller 130 will bedescribed according to an embodiment of the present invention. Theengine controller 130 includes a fire/skip sequence generator 106, afire/skip control unit 110, a power train parameter adjusting module108, and an engine diagnostics module 150. The engine controller 130 isarranged to operate the engine in a skip fire manner. The enginediagnostics module 150 is arranged to detect any engine problems (e.g.,knocking, misfire, etc.) in the engine. Any known techniques, sensors ordetection processes may be used to detect the problems. In variousembodiments, if a problem is detected, the engine diagnostics module 150directs the fire/skip sequence generator 106 to perform operations toreduce the likelihood of the problem arising in the future. Such actionsmay include adjusting the gas spring category on one or more cylindersand/or changing the power train parameters.

The engine controller 130 receives an input signal 114 representative ofthe desired engine output and various vehicle operating parameters, suchas an engine speed 132 and transmission gear 134. The input signal 114may be treated as a request for a desired engine output or torque. Thesignal 114 may be received or derived from an accelerator pedal positionsensor (APP) or other suitable sources, such as a cruise controller, atorque calculator, etc. An optional preprocessor may modify theaccelerator pedal signal prior to delivery to the engine controller 130.However, it should be appreciated that in other implementations, theaccelerator pedal position sensor may communicate directly with theengine controller 130.

The fire/skip sequence generator 106 receives input signal 114, enginespeed 132, transmission gear signal 134 and possibly other inputs and isarranged to determine a fire/skip sequence and gas spring type onskipped firing opportunities that would be appropriate to deliver thedesired output. In various embodiments, the fire/skip sequence is anydata that indicates whether a firing opportunity will be a fire or askip and the type of gas spring associated with the skip. In engineswith multi-level dynamic skip fire control as described in U.S. Pat. No.9,399,964, which is incorporated in its entirety for all purposes,information regarding whether a fire is a low or high output firing mayalso be determined by the fire/skip sequence generator.

Information concerning the fire/skip sequence (or more generally the aircharge sequence if a fire can have a low or high output) is inputted tofire/skip control unit 110 over signal line 116. The fire/skip controlunit 110 orchestrates the opening and closing of the intake and/orexhaust valves to implement the fire/skip sequence and gas spring typeassociated with any skipped firing opportunity. For cam operated valveswith lost motion collapsible type lifters, the fire/skip control unitoperates solenoid control valves located in engine 112. These solenoidvalves control the oil pressure in the collapsible lifter to determinewhether the lifter is in its rigid or collapsible state. In FIG. 8 theengine 112 is depicted has having eight cylinders, but the engine canhave any number of cylinders, such as 2, 3, 4, 5, 6, 8, 10, or 12.

In addition to the fire/skip control unit 110, engine 112 also receivescontrol signals 119 from power train parameter adjusting module 108.Control signals 119 can include, but are not limited to, spark timing,injected fuel mass, throttle blade position, cam phaser position(s), camvalve lift, torque converter slip, exhaust gas recirculation valvesettings, etc. The power train parameter adjusting module providesappropriate adjustment of these various power train parameters to ensurestable combustion for firing cylinders. The combustion conditions, suchas spark timing, may be optimized to provide for maximum fuelefficiency.

In addition to impacting the NVH level, the gas spring type may alsoimpact engine torque and engine fuel efficiency. FIG. 9 compares enginebrake torque versus intake manifold absolute pressure (MAP) for anengine operating at 1600 rpm and an operational firing fraction of ⅓ forAS (without re-intake) and LPES (without re-exhaust) type gas springs.The engine is operating at a stoichiometric air/fuel ratio in bothcases. The LPES torque output 910 is significantly higher than the AStorque output 920 at a given MAP level. This stems, at least in part,from a smaller air charge in the cylinder due to gas leakage from thecylinder while it is acting as an air spring and the lack of re-intakein the AS case, whereas the LPES case has an intake stroke immediatelybefore re-firing.

While the engine output torque is generally lower with AS compared toLPES gas spring types with the aforementioned re-intake and re-exhauststrategy, the fuel efficiency behavior is more complex. FIG. 10 comparesthe engine brake torque versus mass air charge for AS and LPES gasspring types under the same engine operating conditions as FIG. 9. TheLPES curve 1010 shows an almost linear dependence, while the AS curve1020 crosses over at higher MAC values. Since the fuel consumption isproportional to MAC, FIG. 10 demonstrates that fuel efficiency under lowloads is greater using an AS and under high loads using a LPES. Onereason AS has better efficiency at low loads is that AS maintainstraditional valve overlaps whereas LPES does not. As the MAC increases ahigher proportion of the charge is either lost or diluted whileoperating with an AS type gas spring, reducing fuel economy. Thecylinder load threshold for changing gas spring type to maximize fuelefficiency can vary engine speed and firing fraction or firing pattern.

The improvement in fuel efficiency by operating with an AS in certainlow load regimes is clearly demonstrated in FIG. 11. FIG. 11 plots thebrake specific fuel consumption (BSFC) versus the brake mean effectivepressure (BMEP). Inspection of FIG. 11 shows that AS operation (curve1120) results in better fuel efficiency than LPES operation (curve 1110)for loads below approximately 1.5 bar BMEP. The crossing of curves 1110and 1120 near 0.6 BMEP is an artifact of the interpolation and shouldnot be interpreted as LPES being more efficient than AS under these verylow load conditions. Thus, under these loads, and the assumed enginespeed, firing fraction, and re-intake/re-exhaust strategy it ispreferable to operate with an AS and for other loads to operate with aLPES to improve fuel economy.

Aside from NVH and fuel efficiency, the gas spring choice may alsoimpact oil consumption and emissions. When the cylinder pressure isbelow atmospheric pressure, there is a tendency for oil that lubricatesthe cylinder walls to be drawn into the cylinder. This oil will then bepresent for the next combustion event, where it may vaporize andexperience incomplete combustion. The vaporized oil, and its combustionconstituents, then flow into the exhaust manifold during the exhauststroke. If left untreated, the oil and combustion constituents in theexhaust stream may result in undesirable emissions into the environment.

Because of possible oil ingress into the cylinders, it may be desirableto avoid prolonged use of a LPES type spring, since this spring typeoperates with the lowest in-cylinder pressures. One particular situationwhere cylinders may be deactivated for extended time periods is duringdecel cylinder cut-off (DCCO). This operational mode may be used duringdriving conditions when no engine torque is needed, such as coasting,going downhill, or stopping a vehicle. A DCCO event may extend for manyseconds resulting in many successive skipped firing opportunities.During DCCO all the engine's cylinders are deactivated, so each firingopportunity is skipped and the cylinders are not fueled. As a result, noor little air is pumped thru the engine. DCCO operation improves fueleconomy and reduces requirements to rebalance a catalytic converter thatmay be part of an emission control system. The advantages and use ofDCCO operation is more fully described in U.S. Pat. No. 9,790,867, whichis incorporated in its entirety for all purposes.

When no engine torque is requested, it may be desirable to avoidexclusive use of a LPES and instead use at least some AS or HPES typegas springs during the period of no torque request. For example, toreduce use of a LPES type spring in DCCO, an air spring or high pressureexhaust spring type spring may be used for a least one working cycle ofthe succession of skipped working cycles. In one embodiment, a DCCOevent may begin using a LPES in each cylinder and then switch to usingan AS after some time period or number of skipped working cycles. Inanother embodiment, a DCCO event may use a HPES initially on at leastsome cylinders and then shift to an AS at some point in the DCCO event.It should be appreciated that generally the length of a DCCO event isnot known when the DCCO event is initiated, so some DCCO events, such aslonger events, may use different types of gas springs, while other DCCOevents, such as shorter DCCO events, may use only one type of gasspring.

Transitioning from one gas spring type to another gas spring type withina succession of skipped firing opportunities generally involves openingan intake or exhaust valve at the appropriate time during a workingcycle. A HPES will always require a combustion event to generate highpressure exhaust gases. So long as the intake or exhaust valve remainclosed after combustion a HPES will remain in the cylinder (ignoringmass and heat losses, which will, of course, occur). A transition from aHPES to an AS may be made after a desired number of HPES working cyclesby opening first the exhaust valve for one stroke and then the intakevalve for one stroke. A scavenging process will occur and an AS with afresh charge will be present in the cylinder. To prevent combustion,fuel injection and/or spark will need to be deactivated. A transitionfrom a HPES to a LPES after a desired number of HPES working cycles maybe made by opening the exhaust valve for one stroke. Most of the exhaustresidual will be purged from the cylinder, but no new charge will beinducted since the intake valve remains closed. The cylinder may operatewith a LPES type gas spring for as long as desired. At least onecombustion event is required to go back to an HPES from an AS or LPEStype of gas spring.

Transitions between an AS and LPES type gas spring can be made as manytimes as desired, but there is some pumping work associated with theseswitches. To transition from a LPES to AS, the intake valve isreactivated for one stroke at or near TDC to induct a fresh air charge.Air spring operation may be maintained as long as desired by disablingfuel injection and/or spark. To transition from an AS to LPES, theexhaust valve is reactivated for one stroke at or near BDC of anexpansion stroke. This vents cylinder gases, which are mostly air in anAS, into the exhaust system. The intake valve remains closed and thecylinder is now operating with a LPES for as long as desired.

In certain driving situations it may be desirable to use a HPES gasspring type on skipped cylinders. For example, if operation with a fixedset of activated and deactivated cylinders is anticipated for anextended time period, a HPES may be used on the skipped cylinders. Asubstantially constant torque demand for a long duration may beexpected; for example, during cruising on a flat, open road at a firingfraction whose denominator is a factor of the number of enginecylinders; for example, ½ for 4, 6 or 8 cylinder engines, ⅓, ½, ⅔ for 6cylinder engines, etc. Since the HPES is anticipated to occur over anextended number of working cycles, pumping losses associated withventing the HPES will be small. Use of a HPES type gas spring willreduce the possibility of oil ingress into the skipped cylinders andpossible emission issues.

Advantageously, dynamic control of the gas spring type may be used invehicles with autonomous driving controls as described in U.S. patentapplication Ser. Nos. 15/642,132 and 15/849,401, which are incorporatedin their entirety for all purposes. Autonomous vehicle control generallyoffers greater advanced knowledge of future engine torque requests, sothat the length of successive series of skipped working cycles may beknown or estimated at the initiation of the series of skipped workingcycles. This allows determination of an optimum gas spring type or gasspring switching strategy at the beginning of the skipped working cycleseries. Also, in cases where there are no vehicle occupants, autonomousvehicle control relaxes NVH constraints allowing use of different gasspring strategies.

In practice the engine controller 130 shown in FIG. 8, or an enginecontroller with similar functionality, can dynamically select the typeof gas spring used on any skipped cylinder. This determination can bemade on a skipping opportunity by skipping opportunity basis so as tooptimize fuel efficiency and maintain NVH below an acceptable level.Exhaust emissions and oil consumption may also be considered indetermination of the spring type. The type of spring associated with anyskip may be determined algorithmically or may be based on one or morelookup tables that indicate the spring type to be used under particularoperating conditions. It should be appreciated that on any given enginecycle not all cylinders need have the same spring type. That is oneskipped cylinder may be operating with an AS while another skippedcylinder may be operating with an LPES. Also, if a cylinder is skippedon successive firing opportunities the nature of the skip may bechanged, for example, from a HPES to a LPES by venting the cylinder atsome point during the skipping sequence.

This type of control is possible using cam operated valves with “lostmotion” type deactivation. The valve control may be arranged so that asingle solenoid activates/deactivates both the intake and exhaust valvesor that the intake valve and exhaust valve have independentactivation/deactivation solenoids. As described in more detail inco-pending U.S. patent application Ser. No. 14/812,370 there is alatency period between when a fire/skip decision is made and when it isimplemented. This latency period can be in the range of 4 to 12 firingopportunities for cam actuated valves. This latency period allows thefiring control unit 110 sufficient time to activate/deactivate theintake and/or exhaust valves as appropriate. The latency period alsoallows the power train parameter adjusting module sufficient time toadjust power train parameters such as throttle, spark timing, injectedfuel mass, torque converter clutch slip, etc. to provide the requestedengine output with optimum fuel efficiency and an acceptable NVH level.

Many control strategies can be used to determine an appropriate skipfire and gas spring pattern that delivers the requested engine output,while simultaneously minimizing fuel consumption and providing anacceptable level of NVH. For example, short-horizon optimal control,also known as model predictive control (MPC) or receding horizon controlmay be used. Herein short-horizon may refer to optimization of springtype over the queue of firing decisions that have been made, but not yetimplemented. This may be in the range of 4 to 12 firing opportunities,but could be more or less. Since these decisions are known before theyare implemented, the torque output, fuel consumption, and NVH levelsassociated with any fire/skip sequence and corresponding sequence of gasspring types can be calculated. The fire/skip sequence generator cancompare the characteristics associated with any given sequence andselect the best sequence based on optimization criteria. Furthermore,predictions of torque demand and corresponding firing decisions beyondthis horizon can be included in the optimization problem to be solved.

Model-predictive control is a variant of optimal control in which asimplified mathematical optimization problem is solved repeatedly as thesystem is in operation, using the latest sensor information each timefrom the system to counteract the fact that the optimization problem isoften highly simplified and as such only approximates the mathematicalformulation for finding the “best” fire/skip sequence and/or skippingspring types. This combined optimization formulation allows generatingthe requested engine output, while maximizing fuel economy andmaintaining an acceptable NVH level.

In general, optimization is a taxing computational operation that is notguaranteed to converge in a fixed number of iterations. In order to helpthis situation for real-time implementation, a short-horizonoptimization algorithm that implements short-horizon optimizationcontrol can be highly simplified. Since the optimization is solvedrepeatedly as time progresses, errors in the solutions obtained to thesimplified optimization problems relative to the original more complexoptimization problem will, if the simplifications are chosen properly,use current measurements of engine parameters. The use of currentparameters may be considered a type of feedback control to correct thesolution in the direction of the original optimization criteria. Analternative method to reduce the computational overhead of real-timeoptimization is to precompute and tabulate portions of the optimizationproblem. The net result is to trade off computational burden for memoryburden, which in certain electronic control units may be advantageous.

It should be also appreciated that any of the operations describedherein may be stored in a suitable computer readable medium in the formof executable computer code. The operations are carried out when aprocessor executes the computer code. The computer code may beincorporated in an engine controller that coordinates the opening andclosing of the intake and exhaust valves.

The invention has been described primarily in the context of gasolinepowered, 4-stroke piston engines suitable for use in motor vehicles.However, it should be appreciated that the described methods andapparatus are very well suited for use in a wide variety of internalcombustion engines. These include engines for virtually any type ofvehicle—including cars, trucks, boats, aircraft, motorcycles, scooters,etc.; and virtually any other application that involves the firing ofworking chambers and utilizes an internal combustion engine. The variousdescribed approaches work with engines that operate under a wide varietyof different thermodynamic cycles—including virtually any type of dieselengines, Otto cycle engines, Dual cycle engines, Miller cycle engines,Atkinson cycle engines, Wankel engines and other types of rotaryengines, mixed cycle engines (such as dual Otto and diesel engines),hybrid engines, radial engines, etc. It is also believed that thedescribed approaches will work well with newly developed internalcombustion engines regardless of whether they operate utilizingcurrently known, or later developed thermodynamic cycles.

Although only a few embodiments of the invention have been described indetail, it should be appreciated that the invention may be implementedin many other forms without departing from the spirit or scope of theinvention. For example, the control strategies described herein could beimplemented with a fully flexible valve train that is not dependent on acamshaft for valve event timing. Some of the strategies described hereincan also be used in the absence of injection and ignition control, forinstance, on a homogenous charge compression ignition (HCCI), or similarcompression or spark-assisted combustion engines that utilizes premixedor non-pre-mixed air/fuel charges. The fire/skip sequence can also, insome cases, contain information on whether a fire results in either ahigh or low output level. Thus, the sequence may be one of high outputfires, low output fires, and skips. While the invention has generallybeen describe as using an intake and exhaust valve to control inductionand exhaust of a cylinder, a cylinder may have multiple intake and/orexhaust valves and the control strategies may collectively control theirmotion. Therefore, the present embodiments should be consideredillustrative and not restrictive and the invention is not to be limitedto the details given herein.

What is claimed is:
 1. A method of controlling an internal combustionengine having a plurality of cylinders during skip fire operation of theengine, the method comprising: determining that a selected cylinder willbe skipped on a first firing opportunity having an associated firstworking cycle; selecting a first gas spring type associated with thefirst skipped firing opportunity wherein the selection of the first gasspring type is based at least in part on a then current cylinder load;controlling valves associated with the selected cylinder to cause theselected cylinder to operate in accordance with the selected first gasspring type during the first working cycle; determining that theselected cylinder will be skipped on a second firing opportunity havingan associated second working cycle, the second firing opportunity beingdifferent than the first firing opportunity; and selecting a second gasspring type associated with the second skipped firing opportunity,wherein the second gas spring type is different than the first gasspring type.
 2. A method as recited in claim 1 wherein the selection ofthe gas spring type associated with each skipped working cycle isindividually made on a skipping opportunity by skipping opportunitybasis.
 3. A method as recited in claim 1 wherein the first gas springtype is based is a low pressure exhaust spring and the second type ofgas spring is an air spring.
 4. A method as recited in claim 3 whereinthe selection of the second gas spring type is based at least in part onat least one of: a time lapse that has occurred since the operatingoccurrence of the first skipped firing opportunity; a number of skippedworking cycles in the selected cylinder that have occurred since thefirst skipped firing opportunity; or an engine speed.
 5. A method asrecited in claim 1 wherein the first and second selected gas springtypes are each selected from the group consisting of a low pressureexhaust spring, a high pressure exhaust spring, and an air spring.
 6. Amethod as recited in claim 1 wherein the selection of the gas springtype is based at least in part on optimizing fuel economy whilesimultaneously delivering the requested engine output and providing anacceptable NVH level.
 7. A method as recited in claim 1 wherein cams areused to actuate the valves associated with the selected cylinder.
 8. Amethod as recited in claim 4 wherein the valves include intake and/orexhaust valves and the intake and/or exhaust valves remain in a closedposition if a lost motion valve lifter is in its collapsed state.
 9. Amethod as recited in claim 1 wherein the determination of the gas springtype is based on a short-horizon optimization algorithm.
 10. A method asrecited in claim 9 wherein the short-horizon optimization algorithmincludes in the optimization criteria fuel economy and NVH associatedwith the fire/skip sequence.
 11. A method of controlling skip fireoperation of an internal combustion engine having a plurality ofcylinders, each cylinder being configured to operate in a sequence ofworking cycles with each working cycle having an associated firingopportunity, the method comprising: for each firing opportunity,determining whether to skip or fire the associated working cycle; foreach skipped working cycle, individually selecting a gas spring typefrom a plurality of potential gas spring types to use in connection withsuch working cycle such that the gas spring type for each skippedworking cycle is determined on a skipping opportunity by skippingopportunity basis, wherein different gas spring types are sometimes usedfor different skipping opportunities during skip fire operation of theinternal combustion engine.
 12. A method as recited in claim 11 whereinthe potential gas spring types include at least two selected from thegroup consisting of a low pressure exhaust spring, a high pressureexhaust spring, and an air spring.
 13. A method as recited in claim 11wherein the selected gas spring type is based at least in part on a thencurrent cylinder load.
 14. A method as recited in claim 11 wherein theselected gas spring type is based at least in part on a current enginespeed.
 15. A method as recited in claim 11 wherein the selected gasspring type is based at least in part on a current operational firingfraction.
 16. A method as recited in claim 11 wherein air springs areused when the selected cylinder is operating at a cylinder load lowerthan a designated threshold and low pressure exhaust springs are usedwhen the selected cylinder is operated at a cylinder load higher thanthe designated threshold.
 17. A method as recited in claim 16 whereinthe designated threshold varies as a function of at least one of enginespeed and operational firing fraction.
 18. A method of controllingoperation of an internal combustion engine having a plurality ofcylinders, each cylinder being configured to operate in a sequence ofworking cycles with each working cycle having an associated firingopportunity, the method comprising: directing a selected cylinder to notbe fueled and fired during a multiplicity of skipped working cycles thatsequentially follow a fired working cycle of the selected cylinderwithout any intervening firings of the selected cylinder; operating theselected cylinder as a first type of gas spring for a first plurality ofthe multiplicity of skipped working cycles that sequentially follow thefired working cycle of the selected cylinder; and operating the selectedcylinder as a second type of gas spring for at least one additionalskipped working cycle of the multiplicity of working cycles followingthe first plurality of the multiplicity of working cycles, wherein thesecond type of gas spring is different than the first type of gasspring.
 19. A method as recited in claim 18 wherein the first type ofgas spring is a low pressure exhaust spring and the second type of gasspring is an air spring.
 20. A method as recited in claim 18 wherein thefirst type of gas spring is a high pressure exhaust spring and thesecond type of gas spring is an air spring.
 21. A method as recited inclaim 18 wherein the first type of gas spring is a high pressure exhaustspring and the second type of gas spring is a low pressure exhaustspring.
 22. A method as recited in claim 18 wherein the method isperformed when the engine transitions to a deceleration cylinder cut-off(DCCO) operational mode.
 23. A method of controlling an internalcombustion engine having a plurality of cylinders, each cylinder havingassociated valves and being configured to operate in a sequence ofworking cycles, each working cycle having an associated firingopportunity, the method comprising: directing skip fire operation of theengine in which some working cycles are fired working cycles that arefueled and fired and other working cycles are skipped working cyclesthat are not fired; while the engine is operating in a skip fire mannerwithin a first operational region, directing valve actuation such theskipped working cycles that occur during the skip fire operation withinthe first operational region function as air springs; and while theengine is operating in a skip fire manner within a second operationalregion that is different than the first operating region, directingvalve actuation such that the skipped working cycles that occur duringoperation within the second operation region function as low pressureexhaust springs.
 24. A method as recited in claim 23 wherein the firstoperational region is based at least in part on cylinder load.
 25. Amethod as recited in claim 23 wherein the first operational region is aregion of a cylinder load map below an air spring threshold line.
 26. Amethod as recited claim 23 wherein the first operational region includeslower cylinder loads than the second operational region.
 27. A method ofoperating an engine having a crankshaft, an intake manifold, and aplurality of working chambers, the method comprising, during operationof the engine: deactivating all of the working chambers in response to ano engine torque request such that all of the working chambers areskipped through successive working cycles and no air is pumped throughthe working chambers as the crankshaft rotates; and operating each ofthe plurality of working chambers with an air spring or high pressureexhaust spring type gas spring for at least one working cycle of theskipped working cycles.
 28. An engine controller configured to operatean internal combustion engine having a plurality of cylinders in a skipfire manner comprising: a fire/skip sequence generator, wherein thefire/skip sequence generator generates a sequence of skip/fire decisionsand for each skip decision determines a type of gas spring associatedwith the skip.
 29. An engine controller as recited in claim 28 whereinthe type of gas spring is selected from a group consisting of a lowpressure exhaust spring, a high pressure exhaust spring, and an airspring.
 30. An engine controller as recited in claim 28 wherein the typeof gas spring is selected based at least in part on optimizing fuelefficiency.
 31. An engine controller as recited in claim 28 wherein thetype of gas spring selected is based at least in part on reducing NVH toan acceptable level.
 32. An engine controller as recited in claim 28wherein the type of gas spring is selected on a skipping opportunity byskipping opportunity basis.
 33. An engine controller as recited in claim28 wherein for each firing the fire/skip sequence generator determineswhether the firing has a high output or low output.
 34. An enginecontroller as recited in claim 28 wherein the fire/skip sequencegenerator uses a short-horizon optimization algorithm to determine afire/skip sequence and the type of gas spring associated with skippedworking cycles in the fire/skip sequence.
 35. An engine controller asrecited in claim 34 wherein the short-horizon optimization algorithmincludes in the optimization criteria fuel economy and NVH associatedwith the fire/skip sequence.
 36. An engine controller as recited inclaim 28 wherein in response to a request for no torque from theinternal combustion engine the fire/skip sequence generator generates aseries of sequential skips and at least some of these skips use a highpressure exhaust spring or an air spring.
 37. A computer readablestorage medium that includes executable computer code embodied in atangible form and suitable for operating an internal combustion enginein a skip fire manner that is fuel efficient and has acceptable noise,vibration and harshness (NVH) characteristics, wherein the computerreadable medium includes: executable computer code for generating afire/skip sequence to deliver a desired torque, wherein the fire/skipsequence includes a type of gas spring associated with each skippedfiring opportunity.
 38. A computer readable storage medium as recited inclaim 37 wherein the gas spring is selected from a group consisting of alow pressure exhaust spring, a high pressure exhaust spring, and an airspring.